MONKEYS. 45 



other quadrupeds of the continent exhibit similar or analogous 

 structures assisting the free exercise of an arboreal life. The sloths 

 illustrate forms which have been thus modified, and the American 

 porcupines also possess prehensile tails which are unknown in their 

 Old World neighbours. 



No New World monkey possesses either cheek-pouches or 

 callosities, and the ears are bare in the typical forms now under 

 consideration. In most of these monkeys, the hind limbs are longer 

 than the fore limbs ; but in the spider monkeys the latter exceed the 

 hind limbs in length. As regards the hands and feet, the thumb on 

 the whole is less specialised and distinct from the other fingers than 

 in the Old World apes. The great-toe is large and possesses very 

 free powers of movement, but the thumb can hardly be said to be 

 " opposable." In the spider monkeys, the perfection of tail appears 

 to be counterbalanced by the rudimentary condition of the thumb. 

 A well-known form, the Coaita, has no thumb at all ; and the 

 Chameck possesses the merest vestige of this digit. But, strange to 

 say, whilst the thumb in these cases is rudimentary, all the charac- 

 teristic muscles which move it in other forms are represented. Such 

 a fact would appear to point to the degradation of the thumb, and 

 to its functional abrogation and probable disappearance from the 

 hand of the spider monkeys. In due time the thumb-muscles now 

 represented, may also be expected to disappear. It would seem, 

 indeed, as if the " balance of power," which forms a ruling prin- 

 ciple in the world political, was also an important factor in the 

 world of life. The extreme development of the tail to serve the 

 functions of a hand may, perhaps, legitimately enough, be credited 

 with having played a part in the modification of the thumb of these 

 apes. 



In the New World monkeys, the teeth differ in number from those 

 of man and of the Old World apes. Thus, whilst the latter possess 

 thirty-two teeth, the Platyrhines are provided with thirty-six the 

 presence of an additional premolar tooth on each side of each jaw 

 accounting for the increase in number. In respect of their diet, the 

 American monkeys are, for the most part, fruit-eaters, although some 

 species appear to eat insects as a regular part of their dietary. In 

 form, size, and appearance, as well as in many details of their internal 

 anatomy, the New World monkeys exhibit immense variations. Thus, 

 for example, the brain varies greatly in different members of this 

 group, both in respect of absolute size and of relative development. 

 Some are smooth-brained (e.g. squirrel monkeys) ; whilst others 

 (e.g. Sapajous) possess convoluted brains. If the doctrine that com- 

 plexity of brain-convolutions bears a direct relation to the intelligence 

 of the animal be true, we may possibly on this ground account for the 

 remarkable intelligence of the Sapajous, as on the same ground we 



